Ammonium nitrate-aluminum explosive



1964 M. A. PlCClANO ETAL 3,156,186

AMMONIUM NITRATE-ALUMINUM EXPLOSIVE Filed March 17. 1961 A50V 5m 7505/25 E TOR. Bern! g r on i chael A. Picciano United States Patent3,156,136 AWMGNPJM NlTRATE-ALUMHQUM EXPLQSIVE Michael A. Piceiano, MenloEarle, and Bcrtil V. Carlson, Saratoga, Calih, assiguors to GeneralPrecision, loo, a corporation of Delaware Filed Mar. 17, 1961, Ser. No.96,482 6 Claims. (U. 102-28) This invention relates to explosives. Moreparticularly it relates to an explosive in which ammonium nitrateis theprincipal or only oxidizer and aluminum (or other suitable metal) infinely divided form is the principal or only fuel or reducing agent,such explosive being suitable for use in a detonator or other explosivedevice wherein the explosive charge is initiated by an explodingbridgewire.

The exploding bridgewire system of detonation or initiation ofexplosive, propellant or like materials has arisen as an answer to theneed, in military rockets and missiles and in space craft, for aninitiator which does not employ a conventional primary explosive such aslead azide, lead styphnate or a fulminate. The conventional primaryexplosives serve the purpose of detonating a larger body of lesssensitive explosive; of initiating a train of explosive, propellantand/or deflagrating material; or of performing an initiating function insome other like system. However, such primary explosives are overlysensitive and may lead to premature initiation with resultantdestruction of the device.

The exploding bridgewire system has been developed to circumvent theneed to employ a sensitive primary explosive. An explosive bridgewire(or EBW) system employs a wire of very small diameter through which ashort pulse of very high current density is passed, which melts the wireand vaporizes it. The resulting plasma is itself a conductor. A largeamount of heat is suddenly liberated and a shock wave is created, thecombined effect of which is to initiate an adjacent explosive which maybe considerably less sensitive than the conventional primary explosives.Electrical energy to create the necessary pulse of current is stored ina suitable circult, such as a circuit having a bank of condensers whichare charged and then discharged on demand.

Ammonium nitrate-aluminum explosive mixtures (the so-called ammonals)have been used for this purpose. They are advantageous for this purposebecause they are not as sensitive to impact nor to moderate heat as theprimary explosives. However it has been diflicult heretofore to prepareammonium nitrate-aluminum mixtures which are sensitive to an EBW yet areadequately insensitive to certain rigorous tests such as impact testsand exposure to a rather high degree of heat which is short of the heatcreated by an exploding bridgewire. In service such systems (an EBW withan initiating explosive in the form of ammonium nitrate and aluminum)are subject to severe impact, and stray currents may heat the bridgewireto a rather high temperature without exploding it.

It has been ditficult to prepare ammonium nitratealuminum mixtures whichare, (l) initiated by an EBW,

(2) resistant to impact and (3) insensitive to a hot wire. Thus, when itis sought to make the explosive sensitive .to an exploding bridgewire byreducing the particle size of the aluminum the explosive is overlysensitive to impact and/ or to a hot wire. Moreover, ammonium nitrate ishygroscopic and its tendency to absorb moisture causes the explosive todeteriorate and to become insensitive to an exploding bridgewire.

It is an object of the present invention to provide improvements inexplosives. I

it is another obiect of the invention to provide improvements inammonium nitrate-aluminum explosives.

' sieve Particles byFlowing ice It is a particular object of theinvention to provide ammonium nitrate-aluminum explosives which have areduced tendency to deteriorate in storage as compared to suchexplosives heretofore available, which are sensitive to an EBW systemand which are not overly sensitive in other respects, e.g., beingresistant to impact and to a hot wire.

Another particular object of the invention is to provide an EBW devicehaving an ammonium nitrate-aluminum mixture as an initiating explosiveand which avoids at least some of the disadvantages of EBW devicesheretofore used which have employed ammonium nitrate and aluminum as aninitiating explosive.

It is a more general object of the invention to improve upon ammoniumnitrate-metal explosives generally.

The above and other objects of the invention will be apparent from theensuing description and appended claims.

In accordance With the present invention, ammonium nitrate and aluminumare mixed in suitable proportions, each in finely divided condition. Theammonium nitrate is preferably of high purity, e.g., not less than about99.5% NH NO Reagent grade arrunonium nitrate is preferred. The aluminumis in flaked form and contains very little foreign material such asfatty or oily substances, although a thin coating of fatty material ispermissible. it is important that the flakes of aluminum besubstantially devoid of an oxide coating or that they have, at most, avery thin coating of oxide. A pyrotechnic grade of aluminum is preferredwhich is nonpyrophoric, which contains not more than about 1.5% of oilyor greasy matter and which has a substantially oxide-free surface.

A suitable method of making flaked aluminum is to subject aluminum to ashearing action in the presence of an oily or greasy material such as afatty acid (cg, stearic acid or o-lei-c acid) or a hydrocarbon oil. Thisforms an oily or greasy coating on the aluminum particles whichprevents, or which greatly limits oxidation of the aluminum particles.If the oily or greasy coating is not excessive, it will not act toinhibit the effect of an EBW.

The proportions of ammonium nitrate and aluminum may vary considerablye.g., on the basis of 100 parts total, there may be 16 parts or less ofto 25 parts or more of aluminum, the balance being ammonium nitrate.(All parts and percentages herein are by weight based on the finishedmixture, unless otherwise stated.) Preferably about 15 to 20 parts ofaluminum and to 80 parts of ammonium nitrate are employed.

Both the ammonium nitrate and the aluminum should be finely divided, andif they are subjected to careful grinding and milling operations toreduce the particle size and to intimately blend the fine particles aproduct of greater energy output is obtained.

The ammonium nitrate, prior to the final grinding and millingoperations, preferably has a particle distribution as follows (US.Standard 'Mesh):

10% minus 60 mesh plus 140 mesh 55% minus 140 mesh plus 230 mesh 35%minus 230 mesh The flaked aluminum, prior to the final grinding andmilling operations, preferably has a particle distribution according tothe Coulter technique, approximately as follows:

99% greater than 1 micron 4% greater than microns 50% greater than 32microns and 59% less than 32 microns The Coulter technique is describedin an article by Robert H. Berg entitled Electronic Size Analysis ofSub- Through" a Small Liquid Re 3 sistor, published by American Societyfor Testing Materials, Special Technical Publication No. 234 (1958) aspart of a Symposium on Particle Size Measurement, pages 245-255.

Referring to FIGURE 1 of the drawings, a curve is shown plottingparticle sizes (in microns) of the aluminum as abscissae againstpercentages above the stated size as ordinates. The micron dimensionsshown are derived by the Coulter technique and are the radii of spheresof equivalent volume.

The ammonium nitrate is then subjected to separate grinding and then theammonium nitrate and flaked aluminum are placed in a V cone blender withrubber lined steel balls and subjected to blending. 7

These careful grinding" and'inilling' stps are optional and may bereplaced by conventional mixing and blending but, as stated, thepreferred procedure produces a mixture which has a higher energy output.

In addition to flaked aluminum, any other metal may be used which issufficiently reactive with ammonium nitrate, which is in flaked form,which is of suitably small size and which has a surface free (orsubstantially free) of oxide coating. For example, flaked magnesium,zirconium and cerium may be used, also flaked alloys of these and othermetals. Flaked magnesium is more sensitive to a hot wire than flakedaluminum, and certain metals may require a higher energy input to thebridgewire but some of the advantages of the invention are applicable tosuch other metals. However, aluminum is the preferred metal.

We have found that the mixtures of the present invention, moreparticularly ammonium nitrate and aluminum, are sensitive to anexploding bridgewire and are, therefore, suitable for use in an EBWsystem; that they have excellent storage qualities and do not tend,nearly as much as the ordinary ammonals, to deteriorate in storage; andthat the preferred mixtures pass a number of safety tests including animpact test and hot wire tests, so that they are much safer to use withan EBW than ammonial mixtures used heretofore.

The following specific example will illustrate the practice andadvantages of the invention:

Example ].-A mixture was prepared of 20 parts by weight of aluminum and80 parts by weight of ammonium nitrate. The ammonium nitrate was reagentgrade containing not more than about 0.1% impurities and having aparticle distribution (Standard mesh) as follows: 10%

through 65 mesh and retained on 150 mesh; 55% through 150 mesh andretained on 250 mesh; and 35% passing 250 mesh. The aluminum waspyrotechnic grade, flaked aluminum containing not more than about 0.5%of oily or fatty material or of other impurities soluble in organicsolvents. It had a particle distribution (Coulter method) as describedabove. The ammonium nitrate was subjected to grinding and was then addedwith the aluminum to a cone blender along with rubber lined steel ballsand the mixture was subjected therein to milling and blending.

Referring now to FIGURE 2, wherein is shown a testinitiator charged witha mixture of the invention, the device is generally indicated by thereference numeral 10 and it comprises a metal casing 11 within which issecured a header 12. Also within the casing 11 is a metal sleeve 13Within which is a charge 14 of the explosive mixture of the invention.The lower end of the device is closed by a tape 15 which can be readilyruptured. Insulated wires 16 are shown which extend through the header12 and the ends of which are bridged by a bridgewire 17 of very smalldiameter.

As is well known in the art a device of this character,

wherein the Wire 17 is of proper dimensions and composition," isprovided with a. source of electrical energy such as a circuit includinga bankof charged condensers which are caused. to discharge upon commandand which create a high potent-ialfacross the bridgewire 17 and a heavyflow of current therethrough. This meltsand vaporizes the wire andreleases" a large amount of thermal energy and creates a shock wavewhich detonate the charge 141 Numerous test detonators of the typeillustrated in FIGURE 1 have been loaded with the explosive mixture: ofExample 1 and detonated with but very few failures These test initiatorswere detonated by a 2,000 Volt, 1 microfarad source. Loading densitiesvaried from about 0.60 to 0.72 gram per cc. Moreover, such detonatorshave passed safety tests such as the following:

A low voltage test employing a 35 volt D.C., 01' ohm source impedance,such voltage being applied to the bridgewire. No evidence of functioningof the device: exists.

A 500 volt, l microfarad test.

The device, also, safely passes an impact test in which a a free-falling2 kilogram Weight strikes a small quantity" of the ammoniumnitrate-aluminum mixture from a height of 180 centimeters. Also, in athermal stability test, this material will withstand brief exposures totemperatures up to 325 F. without initiation or degradation.

EBW systems with the preferred ammonium nitrate-- aluminum mixture ofthe invention in contact with the bridgewire are mild detonating systemshaving a detona-' tion rate of 3400 meters per second. A 500 milligramcharge produces 5 to 9 mil dents in a steel plate.

FIGURE 3 illustrates a commercial and military device in which theexplosive mixture of the invention is used to initiate a larger chargeof less sensitive explosive.

FIGURE 4 illustrates a commercial or military device in which theexplosive mixture of the invention isused to ignite a gasless mixturesuch as a metal oxide and aluminum, which in turn functions to ignite arocket motor (not shown).

Referring to FIGURE 3, the device is similar to that shown in FIGURE 2and similar parts are similarly numbered. The casing 11 is itselfencased in a body of explosive 20 which is contained in a housing 21.The mixture 14 in the EBW device 10, when initiated by the explodingbridgewire 17, serves to detonate the explosive 20. The explosive 20 maybe any type of explosive such as TNT, which requires detonation toinitiate it.

Referring to FIGURE 4, it will be seen that a portion of the ammoniumnitrate-aluminum explosive of FIG- URE 2 is replaced by a body ofmaterial 25 which may be, for example, a mixture of molybdenum trioxideand aluminum. This mixture is more remote from the bridgewire 17 and isignited by the ammonium nitrate-aluminum mixture 14 when the latter isinitiated by the exploding wire 17. Ignition of the mixture 25 may serveto ignite a core of igniter material within a hollow cylinder of solidfuel in a rocket motor.

It will, therefore, be seen that the present invention provides a noveland very useful explosive mixture and certain novel and very usefulexplosive devices.

We claim: 7

1. An explosive mixture of ammonium nitrate and aluminum, the aluminumconsisting substantially entirely of finely divided flake aluminum, saidmixture being dry, the proportions of ammonium n trate and aluminumbeing such that the said mixture'is initiated and is completely consumedby an exploding bridgewire device when the bridge wire of such device isbrought into contact with said mixture and said wire is caused toexplode by the passage of current therethrough in a short time intervaland at a high current density.

2. The explosive mixture of claim 1 wherein said ammonium nitrate andaluminum are present in the propor tions-of about 75 to 90 parts byweight of ammonium nitrate and about 25 to 10 parts by weight ofaluminum.

3. The explosive mixture of claim 1 wherein said ammonium nitrate andaluminum are present in the propor-' tions of about to parts by weightof'ammoniurn nitrate and about 20 to 15 parts by weight of aluminum- 4.An exploding bridge wire device comprising a housexploding bridgewiretype which, upon passage therethrough of electric current in.

a short interval of time and at a high current density, will explode,and a charge of explosive in contact with said bridge Wire to beinitiated thereby, said charge or explosive consisting substantiallyentirely of a mixture of finely divided ammonium nitrate and aluminum,said mixture being dry and said aluminum consisting substantiallyentirely of flake aluminum, the proportions of ammonium nitrate andaluminum being such that said mixture is initiated and is completelyconsumed by operation of said device by the passage of current throughand resulting explosion of said bridge Wire.

5. The device of claim 4 wherein said ammonium nitrate and aluminum arepresent in the proportions of about 75 to 90 parts by weight of ammoniumnitrate and about 25 to 10 parts by weight of aluminum.

6 6. The device of claim 4 wherein said ammonium nitrate and aluminumare present in the proportions of about to parts by Weight of ammoniumnitrate and about 20 to 15 parts by weight of aluminum.

References Qited in the file of this patent UNITED STATES PATENTS

1. AN EXPLOSIVE MIXTURE OF AMMONIUM NITRATE AND ALUMINUM, THE ALUMINUMCONSISTING SUBSTANTIALLY ENTIRELY OF THE REACTION AREAS AT SPACEDLOCATIONS THEREON SYMMETRICALLY OF ITS TRANSVERSE SYMMETRICAL AXIS, ANDINLET AND OUTLET PORT MEANS IN THE ONE REACTION AREA FOR PORTING FLUIDFOR THE CHAMBERS, WHEREBY ROTATION OF THE FIRST MEMBER ABOUT THE FIRSTROTATIONAL AXIS MOVES THE FIRST REACTION AREA THEREON TRANSVERSELY OFTHE SECOND ROTATIONAL AXIS OF THE SECOND MEMBER TO ADJUST THE OPERATINGSTROKE OF THE HYDRAULIC UNIT EFFECTED UPON ROTATION OF THE SECOND MEMBERAND THE CYLINDER BLOCK ABOUT THEIR RESPECTIVE ROTATIONAL AXES.